The present invention relates generally to the field of capacitors. More specifically, the present invention relates to wet-tantalum capacitors for use in medical devices (e.g., implantable medical devices (IMDs)) or other types of devices.
Since their earliest inception, there has been significant advancement in body-implantable electronic medical devices. Today, these implantable devices include therapeutic and diagnostic devices, such as pacemakers, cardioverters, defibrillators, neural stimulators, drug administering devices, among others for alleviating the adverse effects of various health ailments. Conventional implantable medical devices are also vastly more sophisticated and complex than their predecessors, and are therefore capable of performing considerably more complex tasks for reducing the effects of the health ailments they were designed to treat.
Usually, body-implantable medical devices rely on battery power to perform their therapeutic and/or diagnostic tasks. The battery supplies power to the electrical components of the implantable medical device, and also typically provides power to a capacitor of, for example, a defibrillator device, which stores the energy supplied thereto from the battery. The capacitor typically includes an anode, a cathode, and a fluid electrolyte disposed between the anode and cathode. The various types of materials used for the anode, cathode, and electrolyte may have an impact on the capacitor's ability to store energy from the battery, and the rate at which the energy is stored prior to discharging the capacitor. Importantly, the materials may also affect the volume of the device. Smaller defibrillator devices, which in turn require smaller capacitors, will typically enhance the patient's comfort.
Typically, in an implantable medical device, the capacitor is used to deliver therapeutic electric signals to the patient's heart in response to the device receiving abnormal feedback signals from the heart. The therapeutic electric signals delivered to the patient's heart may vary somewhat in intensity depending on the patients' physiology and the details of the implant. Typically, the therapeutic electric pulse energy delivered to the heart is on the order of 30 J for a single defibrillation pulse. The energy stored in the capacitor has to be somewhat larger due to losses along the delivery path during the release of the energy.
The capacitor, therefore, plays a vital role in the implantable defibrillator device for if the energy supplied from the battery is not stored in a timely manner within the capacitor prior to its therapeutic discharge or if the energy is not released in a timely manner during its therapeutic delivery, the capacitor may not be able to deliver sufficient energy to the patient's heart at a critical point in time when deemed necessary by the implantable medical device. As a result, the performance of the medical device may be adversely affected by the capacitor's inability to adequately and/or quickly store the energy supplied by the battery.
Wet-tantalum capacitors use tantalum and tantalum oxide (Ta2O5) instead of the conventional aluminum and aluminum oxide in aluminum electrolytic capacitors. Wet-tantalum capacitors typically include a tantalum metal anode, a tantalum oxide dielectric combined with a liquid electrolyte that is sandwiched between the anode and a cathode. The cathode may be a tantalum metal using a Ta2O5 dielectric or other material, for example, RuO2.
An exemplary wet-tantalum capacitor may comprise a tantalum metal anode, Ta2O5 dielectric, a liquid electrolyte, and a cathode of material other than tantalum, for example, RuO2. Capacitors of this description are known in the trade as hybrid capacitors, with some versions having tantalum cases and others having polypropylene cases. See also U.S. Pat. Nos. 5,982,609; 5,469,325; 5,737,181; and 5,754,394.
Exemplary specifications for the wet-tantalum capacitors are 185 volts surge, 60 microamp leakage current at 175 volts, 90 microamp leakage current at 185 volts, an AC capacitance of 490 microfarads, and equivalent series resistance (ESR) of 1.2 Ohms. Capacitors meeting these specifications or having similar construction are manufactured by Wilson Greatbatch Technologies of Clarence, N.Y. or Evans Capacitor Company of East Providence, R.I.
A maintenance issue exists for wet-tantalum capacitors, which is that their charging efficiency degrades over long periods of inactivity. The degraded charging efficiency stems from hydration of either the tantalum-oxide dielectric and/or hydration of a sparingly soluble phosphate that is deposited within the interstices of the anode during formation of the anodic oxide. This hydration problem leads to degraded charging efficiency and thereby requires the device battery to expend more energy to charge the capacitor for future use in providing therapy.
Previous attempts at solving the problem of deformation require that the capacitor be charged to a voltage at or near the rated voltage of the capacitor for a period of time and then discharging either through a non-therapeutic load or through leakage currents. In U.S. Pat. No. 6,283,985, Harguth, et al. state they have discovered the need for reforming wet-tantalum capacitors in an implantable cardioverter defibrillator (ICD) and describe a method for doing so, albeit contrary to and without the advantages of the methods and apparatuses described herein.
Accordingly, there is a need for an efficient reformation method and apparatus for wet-tantalum capacitors. Further, there is a need for a method and apparatus for reforming wet-tantalum capacitors which require reduced amounts of energy from a power source. Further still, there is a need for wet-tantalum capacitors which provide improved reformation properties when compared with conventional wet-tantalum capacitor reformation techniques. Yet further still, there is a need for a method and apparatus for reforming wet-tantalum capacitors which addresses the underlying mechanisms that lead to the degradation of wet-tantalum capacitor performance.
It would be desirable to provide a system and/or method that provides one or more of these or other advantageous features. Other features and advantages will be made apparent from the present specification. The teachings disclosed extend to those embodiments which fall within the scope of the appended claims, regardless of whether they accomplish one or more of the aforementioned needs.